Tunable antennas with different operating frequency bands have received increasing attention recently. However, most of them use diodes or shorting pins to achieve tuning performance. This additional circuitry adds protrusion and complexity to the circuit structure that impedes the capability of these antennas to operate in a high temperature, conformal and rugged environment.
The use of ferroelectric materials in phase shifters is disclosed in "Ceramic Phase Shifters for Electronically Steerable Antenna Systems", Varadan et al., Microwave Journal, January 1992, pages 116-126. Some different configurations also appear in U.S. Pat. No. 5,561,407 and U.S. Pat. No. 5,307,033, both issued to Koscica et. al. The use of ferroelectric tunable resonators in filter circuits appears in U.S. Pat. No. 5,617,104 to Das. Ferroelectric materials have also been described for use in electronic phased scanning periodic arrays. For example, such arrays are described in U.S. Pat. No. 5,589,845 to Yandrofski et al., U.S. Pat. No. 5,729,239 to Rao and U.S. Pat. No. 5,557,286 to Varadan et. al. In such arrays, electrical scanning of an RF energy beam pattern is the main concern.
The common dielectric constant values for barium strontium titanate materials used in the systems disclosed in U.S. Pat. No. 5,427,988 to Sengupta et al. and U.S. Pat. No. 5,557,286 to Varadan et al. are relatively high for typical antenna applications. The challenges and difficulties to produce a low dielectric constant material with good electrical properties for antenna applications has been highlighted in "Ferroelectric Materials For Phased Array Applications", Rao et. al., "IEEE Antennas & Propagation Society International Symposium", volume. 4, pages. 2284-2287, 1997. In trying to produce a low dielectric substrate, electrical inhomogeneity, low tunability and poor loss tangent performance are the commonly associated drawbacks. As a result, most of these ferroelectric antennas are realized on a high dielectric constant substrate.
Microstrip antennas with high permittivity substrates suffer from poor efficiency due to the energy loss associated with the excitation of surface wave modes. It has been found that for a single layer ferroelectric antenna with dielectric constant of around 16, the radiating output power from the antenna is lower than the power supplied to the input port. Parasitically coupled antennas may be used to increase the gain, but for these antennas, the performance is optimized at a certain discrete frequency only.
Accordingly, there is a need for a compact antenna that is electrically tunable. There is also a need for such an antenna with a substantial bandwidth and gain.